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sur le développement des zones arides et semi-arides

Titre : Respuestas de las hormigas granívoras al pastoreo en el desierto del Monte : patrones y mecanismos a distintas escalas

Responses of harvester ants to grazing in the Monte desert : patterns and mechanisms at different scales

Auteur :  : Miretti, María Florencia

Université de soutenance : Universidad de Buenos Aires

Grade : Doctor de la Universidad de Buenos Aires en el área de Ciencias Biológicas 2022

Résumé
In the Monte desert, seeds are the main food resource of harvester ants and their abundance and availability are subject to high levels of variability associated mainly with rainfall and grazing. In this thesis, the effects of temporal and spatial variations in seed availability associated with natural processes and anthropic disturbances on seed-carrying ant assemblages and the ecological responses of some harvester ant species to these variations were studied. We evaluated whether seed-carrying ant assemblages respond to environmental change associated with cattle grazing at a local scale and whether this response is consistent at a regional scale (across four localities along a latitudinal gradient in the Monte). Then, changes in the diet of specialist harvester ants Pogonomyrmex mendozanus, P. inermis and P.rastratus in the central Monte in response to temporal (between years) and spatial (between grazing conditions) variations in seed abundance were analyzed. Finally, it was evaluated whether the food preferences for grass seeds of P. mendozanus and P. inermis vary between contrasting grazing conditions in the central Monte. In three of the four Monte localities studied, no differences were recorded in the richness or species composition of the seed- carrying ant assemblages between grazing conditions. Instead, richness and composition varied between localities. The three species of specialist ants presented a mostly granivorous diet in both grazing conditions and during the four sampling seasons. However, some differences between species were recorded : P. inermis and P. rastratus showed a diet consisting mainly of grass seeds, while P. mendozanus had a broader diet that included higher proportions of other items when the abundance of grass seeds decreased. grasses. The food preferences of P. inermis and P. mendozanus for grass seeds did not vary between grazing conditions, supporting the hypothesis that preferences are a stereotyped character in these species. The results of this study indicate that the diversity and composition of seed-carrying ant assemblages seem to be more determined by the conditions of each locality than by the effect of grazing. Although no responses to grazing were observed at the community level, the limitations in the behavioural flexibility of the specialist granivorous species and their strong dependence on grass seeds could cause population reductions if the availability of these resources is reduced due to overgrazing and variations in precipitation.

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